Cast iron is typically produced in cupola or induction fumaces, and generally contain between 2 to 4 per cent carbon. The carbon is intimately mixed with the iron and the form which the carbon takes in the solidified cast iron is very important to the characteristics and properties of the iron castings. If the carbon takes the form of iron carbide, then the cast iron is referred to as white cast iron and has the physical characteristics of being hard and brittle which in certain applications is undesirable. If the carbon takes the form of graphite, the cast iron is soft and machinable and is referred to as grey cast iron.
Graphite may occur in cast iron in the lamellar, compacted or spheroidal forms and variations thereof. The spheroidal form produces the highest strength and most ductile form of cast iron.
The form, size and number distribution the graphite takes as well as the amount of graphite versus iron carbide, can be controlled with certain additives that promote the formation of graphite during solidification of cast iron. These additives are referred to as inoculants and their addition to the cast iron as inoculation. In casting iron products from liquid cast iron, there will always be a risk for the formation of iron carbides in thin sections of castings. The formation of iron carbide is brought about by the rapid cooling of the thin sections as compared to the slower cooling of the thicker sections of the casting. The formation of iron carbide in a cast iron product is referred to in the trade as "chill". The formation of chill is quantified by measuring "chill dept" and the power of an inoculant to prevent chill and reduce chill depth is a convenient way in which to measure and compare the power of inoculants. in cast iron containing spheroidal graphite the power of inoculants is also commonly measured by the number density per unit area of spheroidal graphite particles is the as-cast condition. A higher number density per unit area of graphite spheroids means that the power of inoculation or graphite nucleation has been improved.
There is a constant need to find inoculants which reduce chill depth and improve the machinability of grey cast irons as well as increase the number density of graphite spheroids in ductile cast irons.
Since the exact chemistry and mechanism of inoculation and why inoculants function as they do is not completely understood, a great deal of research goes into providing the industry with a new inoculant.
It is thought that calcium and certain other elements suppress the formation of iron carbide and promote the formation of graphite. A majority of inoculants contain calcium. The addition of these iron carbide suppressants is usually facilitated by the addition of a ferrosilicon alloy and probably the most widely used ferrosilicon alloys are the high silicon alloy containing 70 to 80% silicon and the low silicon alloy containing 45 to 55% silicon.
U.S. Pat. No. 3,527,597 discovered that good inoculating power is obtained with the addition of between about 0.1 to 10% strontium to a silicon-bearing inoculant which contains less than about 0.35% calcium and up to 5% aluminium.
It is further known that if barium is used in conjunction with calcium the two act together to give a greater reduction in chill than an equivalent amount of calcium.
The suppression of carbide formation is associated by the nucleating properties of the inoculant. By nucleating properties it is understood the number of nuclei formed by an inoculant. A high number of nuclei formed improves the inoculation effectiveness and improves the carbide suppression. Further a high nucleation rate may also give better resistance to fading of the inoculating effect during prolonged holding time of the molten iron after inoculation.
From WO 95/24508 it is known a cast iron inoculant showing an increased nucleation rate. This inoculant is a ferrosilicon based inoculant containing calcium and/or strontium and/or barium, less than 4% aluminium and between 0.5 and 10% oxygen in the form of one or more metal oxides. Unfortunately it has been found that the reproducibility of the number of nucleis formed using the inoculant according to WO 95/24508 is rather low. In some instances a high number of nucleis are formed in the cast iron, but in other instances the number of nucleis formed are rather low. The inoculant according to WO 95/24508 has for the above reason found little use in practice.